Development of the Gametophyte Plants

         In the mosses and ferns, the only means of dispersal is the release of the mature spores from the sporangia.  The spores fall to the ground and must develop into independent gametophyte plants.  The early part of this development is particularly susceptible to damage or desiccation in the environment.  In contrast, seed plants do not produce independent gametophyte plants nor does dispersal occur at the spore stage of the life cycle.  The gametophytes of seed plants are very small and are retained within the sporangia throughout their development.  In fact, the female gametophyte plant (or embryo sac) is never released from the parent sporophyte.  The male gametophyte plant, the pollen grain, is only released from the microsporangium during pollination.

         a)            Development of Male Gametophyte

            Within each microsporangium there are many cells called microsporocytes destined to undergo meiosis to produce 4 haploid microspores.  Each microspore then divides mitotically once, producing two nuclei that become encased in a thick wall forming the pollen grain (Figure 13, 14).  One of these nuclei is referred to as the generative nucleus and the other is the tube nucleus.  At this stage, the pollen grains are released from the anthers and are transferred, via the wind or animals, to the stigma of a flower.  This process is known as pollination.  The stigma is specially designed to receive the pollen grain which then germinates.  The tube nucleus produces the pollen tube that penetrates the stigma and grows through the style until it reaches the ovary.  By this time, the generative nucleus has divided once to produce two sperm nuclei.  This three-nucleated structure is the mature male gametophyte.

            Observe  slide #43 which illustrates male gametophyte development.

Figure 13: Development of the Male Gametophyte (Pollen Grain).

 

 

 

Figure 14:  The Stages of the Development of the Male Gametophyte (Pollen Grain), a Microscopic View.

Make a wet mount of the pollen grains from the flowers provided on display and note the sculpted walls of the pollen grains. What is the significance of this sculpting of the walls of the pollen grains?